What is Differential Unit – How it Works-Automobile Study Material


Differential unit

When the vehicle moves straight on
the road, the rear wheels run in the same way as the front wheels. The propeller shaft is connected to follow this. However,when the vehicle is negotiating a turn,the outer wheels cover larger radius than the inner wheels. The outer wheels rotate
more than the inner wheels. Hence the inner wheels have intermittent wear, less grip on the road and withstanding steering issues than outer wheels. The function of the differential unit is to provide different speeds for the inner and outer wheels whenever the vehicle is negotiating a turn. Construction Inside the differential casing sun gears are connected with the rear wheel axle. At the left side of the rear axle differential cage is fixed. For connecting two sun gears planets gears are used. At the end of the rear axle road wheels are connected. The construction of differential unit and the pictorial view of a differential unit can be seen in Figure.

Working principle

When the differential cage rotates,
the planet gears inside the differential casing also rotate along with it. Hence due to the rotation of planet gear, the sun gear which is connected to it also rotates and rotates the right side rear axle. Now due to the rotation of the differential cage, sun gears,planet gears, rear axle and rear wheels all
rotate at the same speed. When the vehicle takes a turn on the road, the planet gear rotates and tends to rotate the outer wheel at high speed and hence the vehicle turns easily on the road.

When the vehicle goes straight

The power passing from the engine
through the clutch, gear box and propeller shaft rotates the pinion shaft of the differential unit. At that time the crown wheel which is connected to the pinion shaft also rotate along with the cage unit. In this condition the planet and sun gears in the differential unit rotate in the opposite direction which creates the brake and the entire unit rotates
with full force. Hence the rear axle shaft which is connected to the sun gear rotates at high speed and transmits the power to rear wheels. In this condition the power is transferred at 90 degree angle and directs the vehicle in a straight direction.

When the vehicle is negotiating a turn

When the vehicle is in a position to take a turn on either right side or left side, the driver rotates the steering wheel according to the turning direction,at that time the load is acting on the axle shaft of the wheel which is going to turn.Hence the sun gear which is connected to it is also subjected to the load and reduces the rotation. At the same time the planet gears which are connected on the top and bottom of it rotate at high speed and rotate the sun gear which is located on other side.Hence the outside wheel rotates at higherspeed than the inner wheel and turns the vehicle for certain distance easily.In this case the speed lost by the inner wheel is gained by the outer wheel and hence the vehicle turns easily on either left or right side.

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